This proposal contains three projects designed to provide a better understanding of the nature of the external forces around the heart. First, I will test the hypothesis that pericardial contact forces vary by region over the surface of the heart in relation to the dynamic volumes of the underlying chambers. I will measure these contact forces with flat balloons and in addition by using sonomicrometry to measure localized areas of the pericardium. In a second portion of this project, I will test the hypothesis that the pericardium is a determinant of early left ventricular filling dynamics. In the second project, I will delineate the interrelationships of the heart and the pericardium during the course of experimental chronic volume overload due to either a systemic arteriovenous fistula or aortic regurgitation. I propose that the pericardium and heart do not grow in parallel. As a result, at some time during the course of volume overload the effect of the pericardium on filling of the heart will be exaggerated. Finally, in the third project I will assess the physiological magnitude of direct diastolic ventricular interaction using a recently developed technique in which cardiac volume is changed over one diastolic cycle. This technique will be applied to assess interaction with chest closed or open and with differing modes and phases of ventilation.